Many industrial applications employ mechanical machinery that utilizes compressed air as a source of power. The use of such pneumatic equipment provides several potential advantages. For example, since the air compressor providing the power may be coupled with the associated pneumatic equipment by way of long air hoses or other conduits, the compressor may be located at a physically remote area, thus resulting in reduced levels of particulate emissions, noise, and other environmental maladies in the immediate area of the pneumatic equipment. Also, with a single compressor possibly powering many different pieces of pneumatic equipment, the overall space consumed by the equipment and the power source combined may be reduced over electrical and other forms of machinery.
Movement of pneumatic equipment is typically accomplished by way of a pneumatic actuator. FIG. 1 provides a simplified cross-sectional diagram of a typical double-acting, single-rod pneumatic actuator 1. In this example, a hollow cylinder 2, capped at each end by an end cap 4 and a head cap 6, provides a vessel into which compressed air may be pumped by way of pneumatic ports 8, 10 and pneumatic channels 12, 14 defined by the caps 4, 6. Within the cylinder 2 resides a piston 16 attached to a rod 18, with the rod 18 extending through an orifice 20 of the head cap 6. Generally, the orifice 20 through which the rod 18 extends distinguishes the head cap 6 from the end cap 4. The end 19 of the rod 18 extending from the head cap 6 may define any of a number of features, such as a set of threads, a square stud, a hole, or the like, to which mechanical machinery may be attached.
In the particular example of FIG. 1, movement of the attached machinery is accomplished by compressed air pumped into the cylinder 2 through either cap 4, 6 via the cylinder ports 8, 10 and channels 12, 14. In response, the piston 16 is moved along the long axis of the cylinder 2, which in turn causes the rod 18 to extend from or retract into the cylinder 2. For specifically, when air is compressed into the cylinder 2 by way of the pneumatic port 8 and channel 12 of the end cap 4, the piston 16 is forced toward the head cap 6, thus causing the rod 18 to extend from the head cap 6. Conversely, if air is forced into the cylinder 2 via the pneumatic port 10 and channel 14 of the head cap 6, the piston 16 is forced back toward the end cap 4, thus forcing the rod 18 to retract back into the cylinder 2.
To provide a substantially airtight compartment formed by the cylinder 2 in the presence of the moving piston 16 and rod 18, a pair of piston seals 22 and a rod seal 24 are typically utilized. In alternative examples, a single piston seal 22 may be employed. The piston seals 22 are essentially rings made of a long-wearing material which prevent compressed air from passing between the sides of the piston 16 and the cylinder 2, thus allowing compressed air entering either the end cap 4 or the head cap 6 to impart maximum air pressure, and thus force, to move the piston 16. Similarly, the rod seal 24 typically is an annular-shaped member sized to allow the rod 18 to fit closely therethrough, thus substantially preventing compressed air from the cylinder 2 from escaping between the rod 18 and the orifice 20 of the head cap 6, thus limiting loss of pneumatic pressure inside the cylinder 2.
In addition, a rod wiper 26 is often included within the orifice 20 of the head cap 6 between the end of the rod 18 and the rod seal 24. Like the rod seal 24, the rod wiper 26 typically is annular in shape so that the rod 18 may slide therethrough. The primary function of the rod wiper 26 is to prevent dust particles and other contaminants from entering and exiting the orifice 20 and the cylinder 2, which could adversely affect the operation and longevity of the actuator 1. Each time the rod 18 is retracted into the cylinder 2, the rod wiper 26 wipes contaminants from the surface of the rod 18, thus preventing the contaminants from reaching the rod seal 24 and other components of the actuator 1.
In contrast to the double-acting, single-rod pneumatic actuator 1 of FIG. 1, several alternative arrangements for pneumatic actuators are also common. For example, single-acting actuators, in which a piston is biased toward one end of a cylinder by a spring, employ compressed air to counteract the force of the spring, thereby requiring only a single cylinder port to allow movement of the piston in both directions along the cylinder. Also, double-rod actuators, as the name implies, employ two rods, each of which is attached to an opposing side of a piston. Thus, one rod extends further from the cylinder while the other is retracted when the piston moves from one end of a cylinder to the other. In addition, single- and double-rod actuators may each be configured as single- or double-acting actuators. Despite the differences among these and other pneumatic actuator arrangements, however, many of the same components depicted in FIG. 1, including the rod 18, rod seal 24 and rod wiper 26, are employed regardless of the arrangement.
One popular environment for the use of pneumatic actuators is a “clean room,” often associated with the manufacture of integrated circuits (ICs). As the name implies, clean rooms provide an environment of greatly reduced levels of dust particles and other contaminants. Production of ICs and other high-technology products normally requires a clean room environment to prevent contamination, which increases product failure rates and reduces production yield.
The use of pneumatic actuators has long been favored for supplying movement for machinery in a clean room due to their low level of negative impact on their local environment, as discussed above. However, as IC geometries continue to be reduced, requiring increased levels of cleanliness during manufacturing, even miniscule levels of foreign material that may be produced during the operation of a pneumatic actuator have become a concern. Using the actuator 1 of FIG. 1 as an example, small amounts of oil or grease commonly used for lubrication within the actuator 1, as well as small particulate matter produced under normal operation of the actuator 1, may escape through the orifice 20 of the head cap 6, past the rod seal 24 and the rod wiper 26 due to the movement of the rod 18 in and out of the cylinder 2. Once such a contaminant has passed the rod wiper 26, the rod wiper 26 may function to sweep the contaminant further down the rod 18, thus introducing small amounts of the contaminant into the clean room environment.
One pneumatic actuator 1a which has been devised in an effort to reduce the contamination is shown in FIG. 2. In addition to the components previously discussed in conjunction with the actuator 1 of FIG. 1, the actuator 1a of FIG. 2 also employs an external vacuum system 30 coupled with the orifice 20 in a void 25 between the rod seal 24 and the rod wiper 26 by way of a vacuum channel 28. The vacuum system 30 operates to remove most contaminants from the void 25 prior to encountering the rod wiper 26, thus reducing the levels of contaminants expelled from the actuator 1a. Unfortunately, supplying the external vacuum system 30 adds significant cost and complexity to an already expensive clean room environment, while typically consuming valuable space.